The Method of Making a High Precision Microelectromechanical Capacitor with Programmable Voltage Source generally relates to monolithic microelectromechanical system (MEMS) devices, and more particularly, to a monolithic device including a MEMS capacitor and a programmable voltage source.
Several techniques are known for adjusting/trimming integrated capacitors within electronic functions. One well-known technique includes removing material by lithography and etching. This method tends to be permanent and must be done as part of the fabrication process. Also, this technique fails to satisfy the requirements of a system that may need continuous and fine-tuned adjustments.
Another well-known technique to create a variable capacitor employs the use of microelectromechanical system (MEMS) capacitors. Traditional MEMS (variable) capacitors include some sort of mechanical or electrostatic actuator to alter the position of the electrodes or the dielectric. The use of MEMS capacitors has several benefits such as, the adjustments are non-destructive, reversible and the MEMS capacitors have high capacitance density-to-size ratios, thereby reducing the overall size of the system.
Traditionally, MEMS capacitors and other analog systems are biased with parameters that are stored off-chip. A traditional analog system 400 is illustrated in FIG. 4. There, analog parameters and biases are physically stored off the analog VLSI circuit 406, by passive components, such as potentiometers 404. This reliance on off-chip biases consumes a number of the chip's pins and thereby limits the complexity of the analog VLSI circuit 406. In addition, potentiometers 404 are relatively inaccurate and susceptible to variations in temperature, humidity, and other physical parameters, such as noise. When used for systems, such as high performance digital-to-analog converters, the inaccuracies of the off-chip biases leads to non-linearities in performance and can limit effective precision.
Recent advances in floating-gate CMOS circuitry have allowed the possibility of integrating highly accurate, stable, and programmable voltage sources into VLSI systems. See Harrison, R., Bragg, J., Hasler, P., Minch, B., Deweerth, S., “A CMOS Programmable Analog Memory-Cell Array Using Floating-Gate Circuits.” IEEE Transactions on Circuits and Systems-II: Analog and Digital Signal Processing, Vol. 48, NO. 1, (January 2001): pp. 4–11, hereby incorporated by reference. Through the use of floating-gate MOS transistors to store charge, voltage potentials may be programmed (set) by the process of tunneling and hot-electron injection.
One of the most complex and difficult aspects of high-performance, SIGMA-DELTA analog-to-digital converters (SIGMA-DELTA ADC) is the inherent non-linearities of the multi-bit digital-to-analog converter in the front-end. The accuracy of a DAC, using a binary-weighted capacitor array, is directly dependent upon the ratio of each capacitor in the array being exactly two times (in capacitance) the capacitor preceding it. The fabrication of each capacitor, however, typically does not allow for a uniformity that is better than a certain tolerance.
Therefore, it can be appreciated that a monolithic device that includes a highly accurate, stable, and programmable MEMS capacitor is needed.